Biomass has been intensely investigated as a renewable feedstock for the production of chemicals and transportation fuels in view of economic, environmental, and political concerns associated with diminishing fossil fuel resources. The development of economically viable processes for the production of chemical intermediates from biomass-derived carbohydrates has become an important challenge for research in this area, such as the development of efficient processes for the production of the platform chemical 5-hydroxymethylfurfural (HMF).1 In this respect, HMF has been identified as a primary building block for the production of furanic polyesters, polyamides, and polyurethanes analogous to those derived from the petroleum polymer industry.2 
Conventionally, HMF is produced from the acid-catalyzed dehydration of C6-sugars (i.e., hexoses). The formation of HMF is proposed to take place through the dehydration of a 5-member monosaccharide ring. Hence, fructose, which contains 21.5% of furanose tautomers in aqueous solution, can be converted to HMF more efficiently than glucose which contains 1% of furanose tautomers in aqueous solutions.3 Accordingly, while considerable efforts have focused on the production of HMF from fructose in high yields,1a, 1c, 1d there remains a long-felt and unmet need for a catalytic system that efficiently converts glucose to HMF in water.
While the dehydration of fructose is catalyzed by strong acids, the challenge for the conversion of glucose to HMF is to find a catalytic system that can selectively isomerize glucose to fructose in tandem with the dehydration reaction. Glucose isomerization proceeds by hydrogen transfer from the C-2 to the C-1 position and from the O-2 to the O-1 position of the α-hydroxy aldehyde to form the α-hydroxy ketone.4 The mechanism of hydrogen transfer from the O-2 to the O-1 position of the aldose is by a proton transfer between the electronegative carbonyl and hydroxyl groups, whereas the transfer of hydrogen from the C-2 to the C-1 position may occur through a hydride ion, or in alkaline conditions as a proton.5 Lewis acids and Brønsted bases are catalysts commonly used for this reaction. However, basic catalysts typically lead to side reactions.6 
Recent studies have been published on developing new processing strategies focused on isomerizing glucose to fructose with a subsequent acid-catalyzed dehydration of fructose to HMF. Lewis acid catalysts in ionic liquids have been shown to be the most promising systems for the conversion of glucose to HMF.1e Zhao, et al. first reported HMF yields of 68-70% in a system consisting of 1-ethyl-3-methyl-imidazolium chloride and a CrCl2 catalyst.1b It is believed that the high activity and selectivity observed for the production of HMF in ionic liquids with chromium catalysts are caused by the stabilization of the transition state for ring opening of glucose by the Lewis acidic Cr center during glucose isomerization.7 Other studies have been carried out on this same reaction using different ionic liquids or Lewis acid catalysts.8 However, the reactions require ionic liquids, which are expensive and subject to deactivation by small amounts of water formed during dehydration reactions.9 Binder, et al. reported that a mixture consisting of dimethylacetamide (DMA), NaBr, and a Lewis acid, CrCl2, could reach HMF yields of 81%, being as effective as ionic liquid systems.1c Huang, et al. reported an HMF yield of 63% from glucose by a two-step process consisting of the isomerization of glucose to fructose in the presence of glucose isomerase and borate ions, followed by the acid catalyzed dehydration of fructose to HMF using HCl as a catalyst in a biphasic system.10 Takagaki, et al. reported HMF yields of 42% at a 73% conversion by a similar two step process by combining a solid acid catalyst, Amberlyst-15, and a solid base catalyst, Mg—Al hydrotalcite, in N,N-dimethylformamide.11 Nikolla, et al. reported HMF yields of 57% at 79% conversion of glucose using a Lewis acidic Sn-Beta zeolite and HCl in a water/NaCl/THF biphasic reaction system.12 